如何用rospy实现ros小海龟在（5.54,5.54），（5.54,8.54），（6.54,7.54），（5.54,6.54），（5.54,5.54）这几个坐标点中循环移动，要求是小海龟先转弯后直行，且能实现闭环控制

首先，需要安装Turtlebot3 package，通过以下命令安装：

sudo apt-get install ros-<ros_version>-turtlebot3

接着，可以创建一个ROS package（假设名称为`turtlebot3_nav`），并在`src`文件夹下创建一个`nav.py`文件，用于控制小海龟移动。

#!/usr/bin/env python
import rospy
from geometry_msgs.msg import Twist
from turtlesim.msg import Pose
from math import pow, atan2, sqrt

class TurtleBot:
    def __init__(self):
        rospy.init_node('turtlebot_controller', anonymous=True)

        self.velocity_publisher = rospy.Publisher('/cmd_vel', Twist, queue_size=10)
        self.pose_subscriber = rospy.Subscriber('/pose', Pose, self.update_pose)
        self.pose = Pose()
        self.rate = rospy.Rate(10)
        
    def update_pose(self, data):
        self.pose = data
        self.pose.x = round(self.pose.x, 4)
        self.pose.y = round(self.pose.y, 4)
        
    def move(self, goal_pose):
        distance_tolerance = 0.1
        vel_msg = Twist()
        
        while self.get_distance(self.pose, goal_pose) > distance_tolerance:
            # Proportional controller
            # Linear velocity in the x-axis
            vel_msg.linear.x = 1.5 * sqrt(pow((goal_pose.x - self.pose.x), 2) + pow((goal_pose.y - self.pose.y), 2))
            vel_msg.linear.y = 0
            vel_msg.linear.z = 0

            # Angular velocity in the z-axis
            vel_msg.angular.x = 0
            vel_msg.angular.y = 0
            vel_msg.angular.z = 4 * (atan2(goal_pose.y - self.pose.y, goal_pose.x - self.pose.x) - self.pose.theta)

            # Publishing our vel_msg
            self.velocity_publisher.publish(vel_msg)

            # Publish at the desired rate.
            self.rate.sleep()

        # Stopping our robot after the movement is over
        vel_msg.linear.x = 0
        vel_msg.angular.z = 0
        self.velocity_publisher.publish(vel_msg)

    def get_distance(self, goal_pose_a, goal_pose_b):
        distance = sqrt(pow((goal_pose_a.x - goal_pose_b.x), 2) + pow((goal_pose_a.y - goal_pose_b.y), 2))
        return distance

if __name__ == '__main__':
    try:
        x = [5.54, 5.54, 6.54, 5.54, 5.54]
        y = [5.54, 8.54, 7.54, 6.54, 5.54]
        turtle = TurtleBot()

        while not rospy.is_shutdown():
            for i in range(len(x)):
                goal_pose = Pose()
                goal_pose.x = x[i]
                goal_pose.y = y[i]
                turtle.move(goal_pose)
    except rospy.ROSInterruptException:
        pass

在主函数中，`x`和`y`分别存储了5个坐标点。在`while`循环中，将每个坐标点作为目标位置传递给`move`函数，小海龟将依次移动到这些位置。

在`move`函数中，通过计算小海龟当前位置与目标位置之间的距离，使用比例控制器计算线速度和角速度，控制小海龟移动。当小海龟到达目标位置时，将停止移动。

最后，需要在终端中运行以下命令以启动ROS节点：

roscore

在另一个终端中，使用以下命令启动Gazebo仿真环境：

roslaunch turtlebot3_gazebo turtlebot3_world.launch

然后，运行`nav.py`文件：

rosrun turtlebot3_nav nav.py

小海龟将在给定的5个坐标点之间循环移动。